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Proton exchange membrane fuel cells (PEMFCs) demonstrate great promise as low emission, high efficiency power sources. However, the sluggish kinetics of the oxygen reduction reaction (ORR) causes large voltage losses at the cathode. There is thus a need for an active ORR catalyst, preferably lower in cost than the most active catalyst known, Pt. Therefore, the primary goal of this work is the development of a low cost, non-noble metal-based ORR catalyst for PEMFC applications. The secondary goal of this work is to better understand and improve the ORR kinetics at these catalysts, as well as to gain insight into the nature of the active site.
The catalysts investigated here have been formed using a modified sol-gel synthesis approach, in which carbon and nitrogen were introduced as specific ligands (ethylenediamine (EN) or o-phenylenediamine (oPDA)) to the metals (Co or Fe) used in the synthesis. After deposition on high surface area carbon supports and heat-treatment, these materials are found to be very active towards the ORR in both acidic and alkaline media. A wide range of methods were used to evaluate the catalysts, including cyclic voltammetry, quartz crystal microbalance, scanning electron microscopy (SEM), x-ray photoelectron microscopy (XPS), Auger Electron Spectroscopy (AES), and more.
It was found that the best results were obtained when using oPDA and Fe as the precursors and then heat-treating at 900 oC for 2 h under N2, resulting in a catalyst only a little less active than Pt, with almost no H2O2 produced. It was also found that using the polymeric analogue of oPDA and supporting the precursors on acid leached carbon instead of as-received carbon gave the best ORR activity in both acidic and alkaline media.
According to this thesis work, it was found that polymeric oPDA, formed either thermally or by pre-polymerization methods, decomposes at pyrolysis temperatures higher than 700 oC, forming ORR-active N sites that may also contain Fe. It was also found from XPS analysis that the N-species are in the form of graphitic and pyridinic sites embedded into the graphene structure, giving very good ORR performance. XPS also showed the presence of Fe2+/3+, which could be chelated to N as FeNx moieties. The microporosity of these types of catalysts was also shown to be essential, where high ORR catalytic activity was achieved for materials with a high micropore surface area. All of these results suggest that the FeNx moieties are hosted and stabilized in the micropores of the carbon support.